Travel to any of the three is very easy from Brussels - Bruges is the longest at just over an hour, Antwerp and Ghent are 43 min and 38 min respectively. I had two days for day trips and I spent one of those days in Bruges, left early and returned late but it was worth the whole long day, so much to see - even with the crowds that everyone hates it was wonderful and as nice as I expected. I actually got a lot in on that one day. Look at RS list of things to do in Bruges with just one day and see if any or all of that appeals to you.
The other day I actually went to both Ghent and Antwerp, only 1/2 day in each but enough to get a feel for the towns, see a couple of important sights (like the Ghent altarpiece), and wander around a bit. Left right after breakfast and was in Ghent by 8:30, was there until about 1:00, then took the train to Antwerp and had lunch and did some sightseeing until about 6:00, then back to Brussels.
I have a tendency to pack a lot in on my day trips and you may not want that fast a pace, but I just wanted to let you know what could be done. You actually could do 1/2 in Bruges and the other 1/2 day in either Ghent or Antwerp but I personally think that, of the three, Bruges deserves a whole day.
Good luck making a choice. You may have to draw straws to decide.